Liquid crystal display (LCD) panels have developed as an alternative to cathode ray tubes (CRTs), offering the advantage of a thin profile and brilliant display. LCD panels have been used for a number of applications, including computer monitors and television displays.
One highly desirable feature for displays is the ability to highlight a portion of a display at a brighter intensity for easier viewing. For example, a computer user may wish to use a cursor to delineate a portion of a picture on a display and brighten that portion for easier viewing. In another example, a computer or television user may want to view one program in the main display and another program in an inset window. The two programs may require different amounts of lighting: a simple, high contrast subject such as text can be easily seen, but a complex subject such as video may require brighter intensity lighting. Highlighting the complex subject makes it easier to see.
LCD panels have lagged CRTs in highlighting functionality. LCD panels typically use one or two fluorescent lamps, such as a mercury vapor cold cathode fluorescent lamps (CCFLs), to provide a uniform backlighting of the LCD panel. CRTs are able to quickly highlight a portion of the display by increasing beam energy, while LCD panel highlighting lags due to the time required to increase the backlight lamp temperature. Individual liquid crystal (LC) elements control the brightness of specific areas of the LCD panel. The lamps must be at the full brightness level before the LC elements can provide the proper highlighting.
For the present generation of LCD panels possessing a highlighting function, the lamps normally operate at 50% lamp current and light output during conditions of non-highlighting. The lamp is stepped to 100% lamp current when highlighting is required. Because of the thermal lag in the lamp, there is visible delay of 10 to 20 seconds before the lamp reaches 100% light output. This is undesirable, as the user must wait for the highlighting to appear. The user may even think that the delay indicates a problem with the display or the computer.
FIG. 1 shows a graph of a step increase in lamp current and the delay in lamp light output. The lamp current is initially 50% and the lamp light output is initially 50%. When the user requests highlighting, the lamp current is increased to 100% and the light output gradually increases from 50% to 100% over 10 to 20 seconds. The user must wait the 10 to 20 seconds before the highlighting is effective.
It would be desirable to have a liquid crystal display panel providing rapid highlighting that would overcome the above disadvantages.